The present invention relates generally to optical inspection systems, and more specifically to auto-focusing techniques for optical inspection systems.
Generally, the industry of semiconductor manufacturing involves highly complex techniques for integrating circuits into semiconductor materials. Due to the large number of processing steps and the decreasing size of semiconductor devices, the semiconductor manufacturing process is prone to processing defects that decrease device yields. Testing procedures to eliminate these processing defects from the processing steps are therefore critical for maintaining high yielding production facilities. Since the testing procedures are an integral and significant part of the manufacturing process, the semiconductor industry constantly seeks more sensitive and efficient testing procedures.
FIG. 1 is a diagrammatic view of a typical optical semiconductor inspection system 100 that includes an integrated auto-focus mechanism 114. FIG. 1 illustrates only the components of an optical inspection system that are relevant to the description of the present invention, therefore various components necessary for operation of the inspection system are not shown. FIG. 1 shows an illumination source 102 that directs light or photons through one or more sets of optical lenses 104 so that a semiconductor wafer specimen 106 can be illuminated and inspected. Optical lens set 104 illustrated in FIG. 1 is an objective lens set. Photons reflected off of specimen 106 are directed back to an inspection detector 108 through beam splitter 110. Inspection detector 108 detects photons reflected off of specimen 106 for inspection purposes. In order for inspection system 100 to produce accurate inspection results, specimen 106 should be positioned within a very small depth of field 112. Otherwise, sensitivity of inspection system 100 will be lost.
To ensure that specimen 106 is within the depth of field 112, auto-focus mechanism 114 is used. Auto-focus mechanism 114 includes a light emitting diode (LED) 116 that directs an auto-focusing light beam 118 into optical lenses 104 such that light beam 118 hits specimen 106, then is directed back into auto-focusing detector 120 via beam splitters 122 and 124. Gratings 126 and 128, which are offset slightly from each other, are placed respectively in front of LED 116 and auto-focusing detector 120. Grating images 126(a) and 128(a) are shown to illustrate the their respective orientations. The position of specimen 106 with respect to field of view 112 affects the intensity of light detected at auto-focus detector 120. Therefore, the focus of optical system 100 can be monitored through auto-focus mechanism 114. Auto-focus device uses at least some of the same optical lens elements in the system as illumination source 102. FIG. 1 illustrates an embodiment where illumination source 102 and auto-focus device 114 both direct light beams through objective lens section 104. Specifically, auto-focus light beam 118 is directed through the field of view of objective lens section 104.
As is commonly known, anti-reflective coatings (AR coatings) are formed on the surfaces of the optical lenses of an optical inspection system. AR coatings beneficially reduce reflections off of the lens surfaces, however, they also tend to reduce the amount of light that can be transmitted through the lenses. As will be described in further detail below, these AR coatings also cause undesirable effects such as reducing the sensitivity of an optical inspection system.
In general, the present invention pertains to techniques for increasing the percentage of light that is transmitted through optical inspection systems that operate in or near the ultraviolet and deep ultraviolet electromagnetic spectrums. Along with increasing the amount of light transmission, the techniques of the present invention also provide additional advantages such as reduction of ripple, increased ability to match inspection systems, and improving manufacturability. The techniques of the present invention involve using an auto-focus light source near the operational range of the inspection system and slightly raising the lower end of the operational range.
One aspect of the present invention pertains to a microscope optical inspection system that includes at least one set of inspection optical lens elements, an illumination source that directs light into the set of inspection optical lens elements, wherein the operational bandwidth of light used for inspection is approximately within the ultraviolet and deep ultraviolet range or within portions thereof, and an auto-focus device that directs an auto-focusing light beam into at least some of the set of inspection optical lens elements, wherein the wavelength of the auto-focusing light beam is proximate to or within the operational bandwidth.
Another aspect of the present invention pertains to a microscope optical inspection system that includes at least one set of inspection optical lens elements, an illumination source that directs light into the set of inspection optical lens elements, and an auto-focus device that directs an auto-focusing light beam into at least some of the set of inspection optical lens elements, wherein the auto-focus device uses a light emitting diode or semiconductor laser made of gallium and nitride.